High efficiency mixing impeller

ABSTRACT

The mixing impeller of the invention has a central, axial hub adapted for attachment to a rotatable driven shaft of a mixer motor. Mounted on the hub is a disc that extends radially from the hub. Circumferentially spaced around the disc are a number of impeller blades. Each blade has two different surfaces for mixing a liquid. One surface is flat and rectangular; the other surface is arrowhead-like, converging from the axial ends of the rectangular surface toward the middle of the rectangular surface of the next adjacent blade. When rotated at a given speed in a fluid in one direction, the impeller draws considerably less power than when rotated in the same fluid at the same speed in the opposite direction. Under certain operating conditions, the impeller has a higher mass transfer efficiency, in both directions of rotation when compared to other impellers. The blades of the impeller may be either open or closed, thereby further modifying the power characteristic of the impeller.

BACKGROUND

This invention relates to a mixing apparatus and method in general, and,in particular, to a mixing impeller with the blades having two differentsurface configurations and a method using such an apparatus.

In many areas of technology, it is frequently desirable to mix a fluidwith one or more other substances. It has been necessary to mix gasses,liquids, and solids, of a powdered or particulate nature, together witha liquid contained in a tank. The mixing process and mixer performanceinvolve the interrelation of three known factors: an impeller, a fluid,and a mixing vessel which may include baffles that contain the fluid.The impeller is used to move and agitate the fluid, and mixing resultsfrom this fluid motion.

Impellers may be divided into two general classifications, axial andradial flow, depending upon their shape, positioning, and vesselgeometry. The flow pattern developed by a mixer will depend upon thegeometric configuration of the impeller and the physical properties ofthe fluid mixture. For examples of various shaped impellers, including apropeller, an arrowhead impeller, and a flat blade impeller, see U.S.Pat. Nos. 2,165,916; 2,384,952; and 2,637,583, respectively.

In a gas-liquid mixing operation, the principal function of the impelleris to disperse a gas stream to an effective bubble size and interfacialarea. At the same time, the impeller should increase the turbulence inthe liquid and provide as uniform as possible distribution of the gaswithin the liquid. The flat blade turbine has proven to be particularlyuseful in achieving the desired gas-liquid mixing.

The flat blade turbine impeller has a disc which is open on both sidesin order to assure good contact for any gas both above and below theturbine level. Hence, it provides good suction above the hub of the discto draw bubbles downward to the impeller and prevents short-circuitingof gas from below the impeller upward along the impeller shaft.Accordingly, the flat blade turbine imparts both an axial flow and aradial flow, thereby maximizing the turbulence of the liquid with whichthe gas is mixed.

The blades of such a turbine are spaced radially from the hub since ithas been found by experiment that little fluid motion takes placeadjacent to the hub of the impeller. In additon, its blades are notshrouded (covered) thereby further contributing to their ability tocreate turbulence as the fluid is subjected to both axial and radialforces generated by the turbine. Such characteristics of impeller design(radial spaced blades, uncovered, and unshrouded blades), served todistinguish mixing impellers from impellers used in other types ofdevices, such as pumps.

In pumps, where turbulence is undesirable, it is a common practice toprovide flow in only one direction (axial or radial). Usually only onesurface of a disc carries blades, and the blades of the pump impellernormally extend from a central hub to the end of the disc in order toseparate and seal a plurality of pump chambers. For an example of onesuch type of pumping impeller, see U.S. Pat. No. 3,136,254. Because oftheir marked differences in purpose and function, pump impellers are notgenerally considered to be useful for mixing operations and vice versa.

During some mixing operations, it is often desirable to vary the powerinput to a mixer motor if the physical properties of the mixture change,e.g. the mixture's viscosity increases. Then the load on the motor mayincrease, and the motor speed will have to be adjusted in order toprevent an overload. In other operations it is desirable to operate themixing process at a different rate. For example, in a fermentationcycle, oxygen is supplied to propagate micro-organisms. At the beginningof such a cycle, the population is minimal and the oxygen demand is low.Hence, a relatively low gas rate and a low mixer speed (low horsepower)is sufficient during the start of such a cycle. However, a higher gasrate and higher mixer speed is needed as the population grows.

In such operations, it has in the past been necessary to provide avariable speed control for the motor or a special two-speed motor andappropriate switchgear in order to accomplish the aforementioned desiredresult. Such variable speed controls and special motors are expensive.Accordingly, it would be desirable to have an inexpensive means foraltering the power required to turn an impeller in a mixing apparatuswithout adjusting the speed of the motor.

In other mixing operations, especially related to mass transfer, such asthe fermentation cycle, the efficiency of the mixing impeller is afunction of mass transferred per unit power consumed. Accordingly, itwould be desirable to provide a more energy efficient impeller for masstransfer operations.

SUMMARY

It is an object of this invention to provide a fluid mixing impellerwhich draws substantially different power depending upon the directionin which the impeller turns.

It is another object of this invention to provide a dual power numbermixing impeller.

It is still another object of this invention to provide a mixingimpeller having blades with two different surfaces facing in oppositedirections.

It is another object of this invention to provide a mixing impeller andmethod for high efficiency for mass transfer operations.

In summary, the invention is for an impeller which comprises an axialhub fixed by any suitable means to a rotatable shaft which may be drivenby any suitable means. A disc is mounted on the hub and extends radiallyoutwardly from the hub. The disc has an upper and a lower surface, bothof which are open and uncovered. A plurality of unshrouded blades aremounted on the disc, circumferentially, end-to-end, and radially spacedfrom the hub. Each blade has two different flow inducing surfaceportions. One surface portion is flat and extends axially and radiallyfrom both faces of the disc and resembles the standard flat blade of atypical flat blade turbine impeller. The other surface portion convergesfrom the axial edges of the flat blade surface to the faces of the discin the form of an arrowhead-like concave configuration. The convergingsurface extends smoothly from the axial edges of one blade to a line onthe disc adjacent the flat surface of the next blade, thereby forming anarrowhead-like tip.

It is a feature of the invention that the impeller, when rotated in onedirection at a given speed, in a given fluid, draws considerably lesspower than when rotated in the opposite direction at the same speed inthe same fluid. The inventive impeller has two embodiments, one in whichthe blade-sections are open and the other in which the blade-sectionsare closed. Both of these embodiments have the same dual power drawcharacteristic, although the absolute power numbers of the two impellers(when compared on the same basis) are different. Accordingly, if themotor driving the impeller begins to overload due to a change in fluidproperties or operating characteristics, the direction of rotation canbe changed in order to decrease the power drawn by the impeller andthereby compensate for the change in the fluid properties.

Prior to developing the invention, it was known that some impellershaving arcuately-shaped blades drew two different levels of power whenrotated in opposite directions at the same speed. Although thisdifference in the power drawn was measurable, it was insignificant withrespect to the power differential required for most mixing operations.Hence, it was an unexpected result that the power numbers for theinvention differ by more than 65%. In addition, it might be expectedthat the power draw would be larger when the impeller was rotated in adirection to present the flat surface as the leading surface. However,as a further result, exactly the opposite occurred. Hence, it is themore streamlined, arrowhead-like surface which draws more power when theimpeller is rotated in the direction in which the arrowhead points.

It is another feature of the invention that the power drawn for aparticular mass transfer requirement is less than the power drawn byflat blade turbines that are typically used in mass transfer operations.

Still another feature is a novel mass transfer method that exhibits arelatively high efficiency for low horsepower inputs.

Having thus summarized the invention, those skilled in the art aredirected to the accompanying drawings and the following detailedspecification.

DRAWINGS

FIG. 1 is a schematic, elevational view of the invention in a mixingvessel.

FIG. 2 is a plan view of the invention.

FIG. 3 is a perspective edge view of the invention with the bladesclosed on their sides.

FIG. 4 is a perspective edge view of the invention with the blades openon their sides.

FIG. 5 is a developed view of the invention.

DETAILED DESCRIPTION

Prior to discussing the invention as shown in the accompanying drawings,it is deemed helpful to the reader to define certain terms which shallbe used throughout the remainder of the specification. To this end, theterm "power number" (Np) is defined as a nondimensional constant whichindicates the relative amount of power needed to drive turbines ofdifferent geometric configurations under identical operating conditions.The term "dual power number impeller" is defined as an impeller in whichthe power number in one direction of rotation is different from thepower number in the reverse direction of rotation. The power numberratio (Npx/Npy) is defined as ratio of the lower power number to thehigher power number when comparing power numbers measure in bothrotational directions. Therefore, the power number ratio is always equalto or less than one.

Referring to the drawings, in particular, FIG. 1, there is generallyshown at 10 a mixing apparatus. Included is a motor 11 for operating adrive 12 which is adapted to turn a rotatable shaft 13. The motor 11 anddrive 12 are suitably suspended above a tank 14 for containing a liquid18. Baffles 15 are suitably provided along the inner surface of tank 14in a manner well known in the art. A sparging pipe 16 passes through thetank 14 for introducing a gas 17 into the liquid 18. The dual powernumber impeller 20 of the invention is fixedly mounted to the end ofrotatable shaft 13. In a manner well known in the art, the sparging pipe16 is adapted to discharge the gas 17 underneath the impeller 20.

With reference to FIGS. 2, 3, and 5, the dual power number impeller 20is shown having an axial hub 21 with an axial opening 22 to accommodatethe rotatable shaft 13. The hub 21 is fixed to the shaft by any suitablemeans. Mounted on hub 21 is a circular disc 25 having an upper face 32and a lower face 33. As shown, the disc 25 is open to the liquid on bothof its faces 32, 33. A plurality of identical blades 26, arecircumferentially spaced around the periphery of the disc 25. Althoughsix blades are shown, any other number of blades suitable to theparticular mixing operation could be used. The blades 26 are radiallyspaced from the hub 21. Each blade includes a flat, rectangular surface27 which is symetrically disposed about the periphery of disc 25. Theaxial length or height of the blade, flat blade surface 27, is the samefrom both the upper and lower disc faces 32, 33. A pair of convergingblade surfaces, 28, 29, extend respectively from the upper blade tip 38and lower blade tip 39 to the middle of the next adjacent flat bladesurface, 26a. The embodiment of the impeller 20 in FIGS. 3 and 5 furtherincludes a pair of oppositely spaced curved inner and outer side walls30, 31 which enclose the volume defined by the flat blade surface 27 andupper and lower surfaces 28, 29. In the embodiment shown in FIG. 4, theside walls 30, 31 are omitted.

EXAMPLES

A number of impellers having a variety of configurations were testedunder standard conditions. These conditions included testing theimpeller in a flat-bottom, vertical cylindrical tank having a diameterof 18 inches and a height of 18 inches. Four standard baffles weremounted equidistant about the inner wall of the tank. The impeller waspositioned 6 inches off the bottom of the tank and was covered with 12inches of water. Gas was introduced through a 6 inch diameter spargeringcentrally located in the tank, 5 inches from the bottom thereof. In eachcase, an impeller was fashioned from a 6 inch diameter disc to which sixblades of various configurations were attached. For each bladeconfiguration, readings were taken of the horsepower supplied to theimpeller at a number of different angular speeds. From such data, thepower number for a given impeller for both directions of rotation wascalculated. By experimenting with a number of different configurations,it was learned that there appeared to be a relationship betweeen the Npratio and the degree of streamlining on one side of the blade whilekeeping the other side flat. On the basis of such experiments, it wasdetermined that the maximum Np ratio could be achieved by an impellerwhich came close to maximizing the streamlining on one side of the bladeand minimizing it on the other. Those results led to the development ofthe two embodiments shown respectively, in FIGS. 3 and 4.

For the configuration in FIG. 3, the radial length of the blades was11/2 inches, the axial length was 11/4 inches each, and the thicknesswas 1/8 of an inch. The configuration of FIG. 3 yielded an Np equal to2.4 in the clockwise direction and equal to 1.4 in the counterclockwisedirection. Hence, the Np ratio for the closed blade design was thelowest of all tested, 0.55. Stated another way, the impeller of FIG. 3drew 55% less power where rotated in the counterclockwise direction.

The impeller shown in FIG. 4 had similar dimensions; the only differencebeing that the blades were open on their sides. There, the power numberin the clockwise direction was 4.2 and in the counterclockwise directionwas 2.6, thus yielding a power number ratio of 0.64.

In another test, the mass transfer efficiency of the impeller 20 of FIG.4 was compared with a similar size flat blade turbine. Mass transferefficiency is generally defined as the quantity of gas absorbed by aliquid per unit horsepower. The test results indicated that at low powerlevels (10 HP per 1,000 gal. of liquid and lower) the impeller 20operated at a higher mass transfer efficiency than did the flat bladeturbine regardless of the direction of rotation. In other words, theimpeller 20 has the capability of producing the same process results asthe flat blade turbine but at a lower horsepower. Hence, at low powerlevels the impeller 20 saves energy due to its higher efficiency. Also,since either power number of the impeller 20 (4.2 or 2.6) is less thanthe power number (5.6) of the flat blade turbine, the impeller 20operates at a higher speed (for a given input horsepower) therebyreducing the torque required for a given operation.

While the foregoing results were obtained with a relatively small scaleimpeller, those skilled in the art are able to fashion larger impellers,see, "Fermentation Mixing Scale-Up Techniques", T. Y. Oldshue, VIIIBiotechnology and Bioengineering, pp. 3-24 (1966).

Thus, having described the invention as well as given details of itspreferred embodiments, those skilled in the art will know that dualpower number impellers can be fashioned by various alterations of theblade geometry without departing from the spirit and scope of thesubject invention as expressed in the following claims.

I claim:
 1. An impeller for agitating liquids comprising a hub, a motorfor rotating the hub in either direction, a disc secured to the hub andextending radially therefrom, and a plurality of similar blades securedto the disc for inducing flow of the liquids when the impeller isrotated in either direction;(a) the blades being longer than they arewide and being disposed circumferentially, end-to-end, about the hub,and axially spaced therefrom, (b) a first end of each of the bladeshaving a relatively flat first flow inducing surface extending in aradial direction relative to the hub for primarily moving fluid when thedisc is rotated in a first direction, and (c) each of the blades havinga second flow inducing surface extending in a plane generally normal tosaid first flow inducing surface for substantially the length of theblade that is concave for primarily moving fluid when the disc isrotated in an opposite direction, (d) whereby substantially less poweris required to rotate the impeller in a given fluid medium in said firstdirection as compared to rotation in said opposite direction.
 2. Animpeller for agitating liquids according to claim 1 wherein the bladeshave internal openings to permit circulation of fluid internally throughthe blades as the impeller is rotated.
 3. An impeller for agitatingliquids according to claim 1 wherein the sides of the blades are arcuatein shape.
 4. An impeller for agitating liquids according to claim 2 orclaim 3 wherein the blades have complementary flow inducing surfacesextending from opposite faces of the disc.
 5. An impeller for agitatingliquids according to claim 4 wherein outer sides of the blades extendsubstantially beyond the outer periphery of the disc.
 6. An impeller foragitating liquids according to claim 1 wherein the sides of the bladesare covered to prevent flow of fluid through the blades internally.